The Haiku operating system uses an extended and modernised version of the Be API that was used by its spiritual predecessor BeOS. Haiku Inc. is expected to drop binary and source compatibility with the BeOS at some point in the future, which will result in its own Haiku API.

The X Window System contains primitive building blocks, called Xt or "Intrinsics", but they are mostly only used by older toolkits such as: OLIT, Motif and Xaw. Most contemporary toolkits, such as GTK+ or Qt, bypass them and use Xlib or XCB directly.

The Amiga OSIntuition was formerly present in the Amiga Kickstart ROM and integrated itself with a medium-high level widget library which invoked the Workbench Amiga native GUI. Since Amiga OS 2.0, Intuition.library became disk based and object oriented. Also Workbench.library and Icon.library became disk based, and could be replaced with similar third-party solutions.

Since 2005, Microsoft has taken the graphics system out of Windows' kernel.[2]

BOOPSI (Basic Object Oriented Programming System for Intuition) was introduced with OS 2.0 and enhanced Intuition with a system of classes in which every class represents a single widget or describes an interface event. This led to an evolution in which third-party developers each realised their own personal systems of classes.

MUI: object-oriented GUI toolkit and the official toolkit for MorphOS.

Windows Forms is .NET's set of classes that handle GUI controls. In the cross-platform Mono implementation, it is an independent toolkit, implemented entirely in managed code (not wrapping the Windows API, which doesn't exist on other platforms).[3]

Note that the X Window System was originally primarily for Unix-like operating systems, but it now runs on Microsoft Windows as well using, for example, Cygwin, so some or all of these toolkits can also be used under Windows.

GTK+, open source (LGPL), primarily for the X Window System, ported to and emulated under other platforms; used in the GNOME, Rox, LXDE and Xfce desktop environments. The Windows port has support for native widgets.

IUP, open source (MIT), a minimalist GUI toolkit in ANSI C for Windows, UNIX and Linux.

Xwt, Mono's toolkit that maps API calls to native platform calls of the underlying platform. In this way it exposes one unified API across different platforms and makes possible for the graphical user interfaces to have native look and feel on different platforms. Runs on Linux, MacOS and Windows.

The Standard Widget Toolkit (SWT) is a native widget toolkit for Java that was developed as part of the Eclipse project. SWT uses a standard toolkit for the running platform (such as the Windows API, OS X Cocoa, or GTK+) underneath.

Qt Jambi, the official Java binding to Qt from Trolltech. The commercial support and development has stopped[4]

IP Pascal uses a graphics library built on top of standard language constructs. Also unusual for being a procedural toolkit that is cross-platform (no callbacks or other tricks), and is completely upward compatible with standard serial input and output paradigms. Completely standard programs with serial output can be run and extended with graphical constructs.

fpGUI is created with the Free Pascal compiler. It doesn't rely on any large 3rdParty libraries and currently runs on Linux, Windows and Windows CE. A Carbon (OS X) port is underway.

CLX (Component Library for Cross-platform), used with Borland's Delphi, C++ Builder, and Kylix, for producing cross-platform applications. It is based on Qt, wrapped in such a way that its programming interface is similar to that of the VCL toolkit.

^Migrating to client-side windows: “GDK looks for the GDK_NATIVE_WINDOWS environment variable and makes all windows native if it is set. It also tries to be more compatible with the way prior versions worked in some other ways.”

^WxWidgets Compared To Other Toolkits: “Qt doesn't have true native ports like wxWidgets does. Qt does not use system provided widgets, but emulates it with themes. What we mean by this is that even though Qt draws them quite realistically, Qt draws its own widgets on each platform.”

^"Qt Modular Class Library". Digia. Archived from the original on 2013-11-01. Qt uses the native graphics APIs of each platform it supports, taking full advantage of system resources and ensuring that applications have native look and feel.

^"The Qt 4 Style API". The Qt Company. Qt's built-in widgets use [QStyle] to perform nearly all of their drawing, ensuring that they look exactly like the equivalent native widgets.